High damping structure

ABSTRACT

A high damping device combined with the frame of a building to protect the building from seismic shock. For seismic vibration up to a predetermined level corresponding to the permissible strength of the high damping device, a damping coefficient c of the high damping device is set so as to be c 3  =c=c 1  with respect to a damping coefficient c 3  for giving the maximum value of a damping factor h 3  corresponding to a tertiary mode of vibration of the structure and a damping coefficient c 1  for giving the maximum value of a damping factor h 1  corresponding to a primary mode of vibration. The maximum load on the high damping device is predetermined and means are provided to prevent the high damping device from being damaged in the event that the predetermined maximum load is exceeded. The inventive combination permits the stiffness factor of the building to be reduced from a factor of 1.0 down to a factor as low as 0.3, with a proportionate reduction in steel frame mass.

This is a continuation of application Ser. No. 07/861,842, filed Jun.17, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of invention comprises devices for damping vibrations ofstructures caused by seismic shock or the like.

2. Description of Related Art

A variety of active and passive seismic response control systems areknown, including variable stiffness devices, to provide for the safetyof structures. For instance, a variable stiffness earthquake-resistingmechanism may be integrated in a column-and-beam type frame structure inthe form of an adjustable brace in which the rigidity of the variablestiffness mechanism, or the means of connection between the frame andthe variable stiffness mechanism, functions to analyze seismicvibrational forces and to provide damping to offset these forces.

Prior art active seismic response control systems attempt to deal withseismic vibrations by actively shifting the natural frequency of thestructure against the predominant period of a seismic vibration.However, seismic motion is an irregular vibration which does not have aclear predominant period, and in some instances, the predominant periodis plural. Furthermore, in the case of prior art active seismic responsecontrol systems, various sensors as well as a controlling computer areused. To safeguard against the possibility of unforeseen events, avariety of safety maintenance mechanisms are necessary, the control ofwhich becomes complicated. These safety mechanisms are not only costly,but require valuable start-up time to become effective. During thisstart-up period, the structure is either unprotected or not fullyprotected.

For instance, Kobori et al. U.S. Pat. No. 4,890,430 discloses an activedamper which is computer controlled to vary the natural resonance of anentire building by actively varying the rigidity of selected structuralmembers. Kobori. et al. U.S. Pat. No. 5,022,201 is an active seismicdamper comprising a mass damper mounted on the top of a building. Thedamper is actively vibrated by an actuator connecting the mass to thebuilding. Ishii et al. U.S. Pat. No. 5,025,599 discloses a combinationactive and passive damping device wherein a mass damper is renderedactively vibratable by a hydraulic actuator. In the event of a powerfailure, the device is converted to a passive damper wherein the mass ispassively vibratable by coiled springs between the mass and the buildingwhich are excited solely by the energy of seismic vibration.

SUMMARY OF THE INVENTION

As used in this specification, the following definitions shall apply:

1. Active damper shall mean a seismic vibration damping device which, inorder to function, requires an actuator energized by means other thanthe energy of seismic vibration.

2. Passive damper shall mean a seismic damping device which functionswithout an actuator and is energized solely by the energy of seismicvibration.

3. Actuator shall mean a mechanical, electro-mechanical, electrical,and/or electronic means for energizing an active damper.

4. Control force shall mean the force applied by a seismic vibrationdamping device to a structure to damp seismic vibrations in thestructure.

5. Fail safe means shall mean a device to automatically deactivate aseismic vibration damper to protect the damper from damage due tooverload or malfunction.

6. Column and beam shall mean state of the art construction materialsused to form the vertical and horizontal frame portions of a structure.

The basic concept of the invention is to use a rigid frame structurewith a stiffness factor of approximately one-half of the stiffness andstrength factor of a frame required in a normal design. To compensatefor the reduced rigidity of the frame structure, the damping devices, incombination with earthquake-resisting elements, such as braces, aresecured to the column-and-beam frame of the structure. Maximum dampingcapacity is obtained for the structure, and the response of thestructure is minimized by preliminarily setting the damping coefficientof the inventive high damping device at a proper value. Although astructure having one-half of the frame stiffness of a prior artstructure is an example of a structure suitable for protection by theinventive high damping device, the invention provides effectiveprotection for column-and-beam frames having a stiffness and strengthfactor substantially within a range of 0.3 through 1.0 of the stiffnessof a prior art structure designed and equipped with prior artearthquake-resisting devices. In the case where the structure stiffnessfactor exceeds 1.0, seismic response reduction becomes progressivelyless effective. On the other hand, where the strength of the structureis less than 0.3, effective damping becomes substantially impossiblebecause of the shearing forces to which the column-and-beam frame issubjected.

According to the present invention, earthquake-resisting braces areprovided within a predetermined column-and-beam type frame of astructure. Either the column-and-beam frame and the braces areinterconnected with the inventive high damping devices, or only thebraces are interconnected by the inventive high damping devices, whichare capable of giving a damping coefficient c within a predeterminedrange, including a damping coefficient for minimizing the response ofthe structure to an earthquake.

With reference to the damping coefficient c of the high damping device,a damping factor of each vibration mode of the structure is obtained bythe following formula (1):

    H.sub.1 =-Re (λi)/|λi|     (1)

wherein

λi:an i-th complex natural value

hi:an i-th damping factor, and

Re(λi):a real number part of the i-th complex natural value.

The damping coefficient c of the high damping device is taken as beingset in the neighborhood of such damping coefficients c₁, c₂ and c₃ whichgive the maximum values of damping factors h₁, h₂ and h₃ correspondingto the primary through tertiary vibration modes, respectively.

With reference to the damping capacity of the structure, a mostadvantageous condition can be obtained by setting the coefficient cwithin the range:

    c.sub.3 ≦c≦c.sub.1

The damping coefficient c of the high damping device is preliminarilyset in the neighborhood of the damping coefficients c₁, c₂ or c₃ (e.g.,25 cm/sec) which provide the maximum values of the damping factors h₁,h₂ and h₃ corresponding to the primary through tertiary vibration modesas described above, and the damping coefficient of the high dampingdevice is preset for the seismic motion at a predetermined vibrationlevel.

Seismic response control can also be accomplished by defining thedamping coefficient c as c_(a) =F_(a) /V_(L') wherein F_(a) is thepermissible strength of the high damping device and V_(L) is a responsevelocity of the high damping device due to an earthquake at apredetermined vibration level. The damping coefficient c of the highdamping device may also be expressed as c_(x) =F_(a) /V_(x) whereinF_(a) is the permissible strength of the high damping device and V_(x)is a response velocity of the high damping device due to an earthquake)for an earthquake at the preceding predetermined vibrational level.

Assuming that the permissible strength F_(a) of a single high dampingdevice is 100 tons, maximum damping is provided for the structure whilekeeping the damping coefficient c at a predetermined constant value of25 tons per cm/sec responsive to seismic vibration up to a level of 25cm/sec, for an earthquake having the maximum speed standardized to 25cm/sec. The load is kept at approximately 100 tons of the permissiblestrength by gradually decreasing the preset damping coefficient from 25cm/sec. Within these parameters, damping can be provided for thestructure within the capacity of the device while at the same timeprotecting the device from damage should the seismic vibrations exceedthe capacity of the device. It is desirable that the damping coefficientc_(a) within a predetermined vibrational level be within the range of c₃through c₁. When the damping coefficient c_(a) falls below this range,the damping effectiveness decreases. Also, when the damping coefficientc_(a) exceeds this range, it becomes difficult to design a high dampingdevice having the capacity to damp such high energy vibration loads.

OBJECTS OF THE INVENTION

It is among the objects of this invention to provide a passive seismicresponse control mechanism which does not need as part of the controlsystem a computer program or the like; to permit a structure to have ahigh damping capacity by properly connecting an earthquake vibrationresisting element, such as a brace, to a high damping device, which arethen interconnected within a column-and-beam type frame structure; toreduce the vibrations of a structure due to external disturbances suchas earthquake, wind, or the like; and to provide safe living spacewithin the structure during a seismic disturbance.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the invention will beapparent from the following description of preferred embodiments of theinvention with reference to the accompanying drawings, in which:

FIG. 1 is a schematic front elevational view of a structure equippedwith the inventive high damping devices;

FIG. 2 is a schematic front elevational view of a prior art structure;

FIG. 3 is a schematic diagram of a vibration model of one-story of astructure protected with the present invention;

FIG. 4 is a graph showing the relationship between the primary throughtertiary damping factors of a column-and-beam type frame structureprovided by primary through tertiary damping coefficients;

FIG. 5 is a graph comparing responses to vibration of a prior artstructure and a structure protected with the inventive high dampingdevice;

FIG. 6 is a graph showing the relation between a seismic load and theeffect of the inventive high damping device;

FIG. 7 is a graph showing the relation between a seismic load and thevelocity of the inventive high damping device;

FIG. 8 is a basic schematic sectional view of the inventive high dampingdevice;

FIG. 9 is a cross-sectional view taken along line 9--9 of FIG. 8;

FIG. 10 is a schematic sectional view of another preferred embodiment ofthe inventive high damping device;

FIG. 11 is a sectional view of yet another embodiment of the inventivehigh damping device;

FIG. 12 is a sectional view of a pressure regulating valve used in apreferred embodiment of the invention;

FIG. 13 is a sectional view of a relief valve used in a preferredembodiment of the invention;

FIG. 14 is a sectional view of a by-pass line and an accumulator deviceused in a preferred embodiment of the invention;

FIG. 15 is a schematic elevational fragmentary view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device secured to a system of inverted V-braces;

FIG. 16 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device and U-shaped connecting brace members;

FIG. 17 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device connected to a seismic shock-resisting wall typebrace and with the inventive high damping device oriented in ahorizontal mode on the top edge of the seismic shock-resisting wall typebrace;

FIG. 18 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device secured between a beam and the foundation of thestructure;

FIG. 19 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device in a horizontal mode secured to a system of crossbraces;

FIG. 20 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device secured to a system of cross braces, similar to FIG.19, but with the inventive high damping device in a vertical mode;

FIG. 21 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device secured to a seismic shock-resisting wall brace,similar to FIG. 17, but with the inventive high damping device securedto a vertical side edge of the seismic shock-resisting wall brace; and

FIG. 22 is a schematic fragmentary elevational view of an embodiment ofa post-and-beam type frame of a structure equipped with an inventivehigh damping device, similar to FIG. 19, but with the cross bracesextended so as to secure a plurality of stories of the structure with asingle inventive high damping device.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring first to FIG. 1, therein is shown a structure 1 employing theinventive high damping device 10, having a column-and-beam type framewhich requires approximately only one-half of the columns 2 required ofa conventional prior art structure, such as shown in FIG. 2, having thesame number of stories. Inverted V-type braces 4, functioning asearthquake-resisting elements, and high damping devices 10 are locallyinstalled at each floor level 3 to absorb vibrational energy impactingthe structure.

FIG. 3 schematically shows a single story model of the inventive highdamping device, in which c is the damping coefficient of the device,K_(F) is the stiffness of the column-and-beam frame, and K_(V) is thestiffness of the brace. With this model, the natural value of amulti-story building can be obtained, and a damping factor for each modeof the structure can be calculated by formula (1), set forth above.

The graph of FIG. 4 shows the relation between the damping factor h(%)of the frame's natural period and the damping coefficient c(tons/cm/sec) of the inventive high damping device 10 for each floorlevel of the structure with respect to primary through tertiarycoefficient modes and their corresponding damping factors. If thedamping coefficient c of the inventive high damping device 10 is setwithin the range a where each damping factor h₁, h₂ or h₃ falls withinthe range of 10 through 40%, a sufficient response reduction effect toseismic vibration can be obtained. Within this range a, the differencebetween the peak of the tertiary damping factor h₃ and the peak of theprimary damping factor h₁ is significant, since it is advantageous toobtain both a damping coefficient c₃ which obtains the maximum value ofthe damping factor h₃ for the tertiary mode and a damping coefficient c₁which obtains the maximum value of the damping factor h₁ for the primarymode, and to set the damping coefficient c of the high damping device asc₃ ≦c≦c₁.

If the damping coefficient c is less than c₃, the deformation of theframe rapidly increases. On the other hand, if the damping coefficient cis more than c₁, there is not much difference in the vibration controleffect, although the strength required for the high damping deviceincreases.

The graph of FIG. 5 illustrates the response reduction effect observedon the basis of a seismic response spectrum. By approximately halvingthe column-and-beam frame natural period (T₁) of a prior art structure,the natural period (T₂) is extended and the spectrum itself is lowered.At the same time, since the damping effect increases by approximately 2%up to 10 through 40%, the response spectrum is further lowered and thenatural period is slightly shortened, as shown at T₃. At this time, theincrease of the structural deformation, which normally becomes aproblem, can be controlled because the damping effect increases.

With reference to the foregoing discussion of FIGS. 4 and 5, thepermissible strength of the high damping device should be taken intoconsideration as well. Thus, since the load applied on the inventivehigh damping device is roughly proportional to the scale and velocity ofthe seismic vibrations when the damping coefficient c is constant, whenthe damping coefficient c is decreased responsive to an earthquakeexceeding a predetermined level (e.g., 25 cm/sec), the applied load willdecrease to a level commensurate with the designed strength of theinventive high damping device.

FIGS. 6 and 7 are graphs showing effects of load on an inventive highdamping device. FIG. 6 shows the relationship between load anddisplacement against a sine wave expressed as F=c_(V), wherein F is aload applied on the inventive device, c is the damping coefficient(tons/cm/sec) of the device, and V is the velocity (cm/sec) of thedevice in response to an earthquake. Displacement of the inventivedevice in response to a level 25 cm/sec earthquake is indicated by theδ₂₅ arrow. Displacement of the inventive device in response to a level50 kine earthquake is indicated by the δ₅₀ arrow. FIG. 7 shows therelationship between load and velocity, and both figures indicate anupper load limit of 100 tons. It is found that the damping coefficient cof the inventive device decreases from a velocity of V₂₅ or adisplacement of δ₂₅ in response to an earthquake at a level of 25cm/sec.

By way of example, assume a twenty-four story building, having a rigidsteel frame structure 98.1 m in height, 3.90 m in typical floor height,and 1269 m² in typical floor area, and assume that the maximum velocityamplitude of the input seismic motion is at a level of 50 cm/sec. Alsoassume that four inventive high damping devices are required on everyfloor of the building in order to have the required strength in theevent of seismic loads in the order of 200 tons. The damping coefficientc is set at 25 tons/cm/sec in order to limit the maximum load to under100 tons applied to each inventive high damping device, and the dampingcoefficient c is decreased against earthquakes exceeding the 25 cm/seclevel so as to avoid harmful increase of the load on each of theinventive devices per se. Thus, in the inventive high damping device therelationship between a load F and a velocity V produced on the highdamping device approaches linearity.

As an embodiment of the inventive high damping device 10, FIG. 8 showsits basic structure, wherein a piston 12, with piston rods 12a and 12b,is incorporated within a cylinder 11. Pressure regulating valves 17a and17b provide two-way flow paths through the piston 12 to enable oil toflow freely between hydraulic chambers 14a and 14b, depending on whichhydraulic chamber is under the greater positive pressure.

In order to protect the inventive device against overload (e.g., inexcess of the predetermined level), relief valves 27a and 27b areprovided in piston 12. When a pressure in excess of the designed load isapplied, either relief valve 27a or 27b opens to release the pressure.In installations in which overload cannot occur, the relief valves 27aand 27b may be eliminated.

FIG. 9 shows the arrangement of pressure regulating valves 17a and 17band relief valves 27a and 27b, which are uniformly circumferentiallyarrayed to form passageways through piston 12.

FIG. 10 diagrammatically shows an embodiment of the damping device 10 inwhich the piston rod 12a projects from the cylinder 11 only in onedirection, and fastening rings 15 and 16 are provided for connecting theinventive high damping device to portions of a frame, such as shown inFIGS. 15 through 22. The high damping device of FIG. 10 includes an oilaccumulator 18 in combination with check valves 20a and 20b so that thedamper will have an adequate supply of oil at all times.

The embodiment of FIG. 11 shows in section pressure regulating valves17a and 17b which are provided within the piston 12 for the purpose ofpreventing oil from leaking to the exterior of the damper. The pressureregulating valves 17a and 17b are provided with conical poppet valves toprovide damping independent of temperature. See also FIG. 12. Fordurability and reliability, multi-stage metal seals 29a are used to sealthe piston 12 for sliding contact with cylinder 11. Two-stage metalseals 29b are also used as fixed seals. In addition, seals 29c, made ofa fluorocarbon resin, are provided in two stages for the rod portion,and the seal 29c on the external side is replaceable as a cartridge.With this combination of sliding and fixed seals, a high dampingcoefficient becomes possible by minimizing the potential for highpressure oil leaks in the system. A three-directional rotatable clevismay be used for connecting the fitting ring 15 to a frame member.

Referring to FIG. 13, therein is shown, in an open-pressure setting,spring 28 in relief valve 27. When the seismic vibration of anearthquake exceeds a predetermined level of energy, resulting inpressure at an inflow portion of the total surface of a valve reaching apressure higher than a designed pressure, the relief valve 27 has apressure pad 27a for opening the valve against the resistance of thespring 28 to release the pressure.

FIG. 14 shows by-pass line 19 and the accumulator 18 which are mountedon the surface of the casing 11 of the high damping device 10. A checkvalve 20a, for preventing an oil flow toward the side of the hydraulicchamber 14a, is provided between the hydraulic chamber 14a and theaccumulator 18, and a check valve 20b for preventing an oil flow towardthe side of the hydraulic chamber 14b is provided between a hydraulicchamber 14b and the accumulator. Moreover, check valves 20a and 20b areattached to orifices 21a and 21b, respectively, passing through each ofthe check valves (in parallel with each other as shown in FIG. 10) tolinearize the damping characteristics of the high damping device 10 andto relieve a pressure build-up within either hydraulic chamber 14a or14b.

FIGS. 15 through 22 show installation embodiments of the high dampingdevice 10 within a column-and-beam type frame.

In the embodiment of FIG. 15, the high damping device 10 is interposedbetween a column-and-beam frame 31 and an inverted V-type brace 35,which functions as the earthquake-resisting element.

The embodiment shown in FIG. 16 employs U-shaped braces 41 which act asearthquake-resisting elements. The high damping device 10 is securedbetween the U-shaped braces 41, which are secured to beams 34 and extendvertically therefrom.

In the embodiment of FIG. 17, the high damping device 10 is interposedbetween the upper beam 34 and an earthquake-resisting wall brace 42.

In the embodiment shown in FIG. 18, the high damping device 10 issecured between the lower beam 34 and the base B of a structure mountedon base isolation pads 43. The earthquake-resisting element is aninverted V-type brace 35, similar to the brace shown in FIG. 15.

In the embodiment of FIG. 19, an earthquake-resisting X-type brace 44 isinstalled within the column-and-beam frame 31. The high damping device10 is horizontally secured at the center of the brace.

In an embodiment similar to that of FIG. 19, the embodiment shown inFIG. 20 comprises the high damping device 10 vertically secured to anX-type brace 45.

In an embodiment similar to that shown in FIG. 17, the embodiment shownin FIG. 21 discloses the high damping device 10 interposed between thebeam 34 and an earthquake-resisting wall brace 46, wherein the highdamping device 10 is secured to the vertical edge of the wall brace 46and over a doorway 47.

In the embodiment shown in FIG. 22, the high damping device 10 ishorizontally interposed at the center of an X-type brace 48 whichextends over three stories of a structure, from floor 49A to floor 49D,with the extremities of the X-type brace secured only to floors 49A and49D.

POSSIBILITY OF INDUSTRIAL UTILIZATION

The following advantages will be obtained by applying a high dampingdevice of the present invention to buildings which are at risk to theravages of earthquakes and high winds.

1. Since the number of columns of a column-and-beam structure can bereduced by approximately 50%, not only is the saving in structural steelconsiderable, but the additional unobstructed floor space betweencolumns considerably increases the floor planning possibilities.

2. Since the response of the structure to earthquake shock and highwinds is reduced, the safety of the occupants and of the structure isincreased.

3. Since the invention is a passive type damper mechanism, only finetuning adjustments to the particular characteristics of the structureare required when installed.

4. Since complicated active seismic control systems and attachedfacilities are not required, installation costs are low in comparison tothe costs of active seismic response control mechanisms.

5. The effective load applied to the inventive high damping device maybe decreased by reducing the damping coefficient for seismic vibrationto a predetermined safe level.

6. The number of inventive damping devices to be installed on each floorof a building can be predetermined.

7. Since the designed load limit of the inventive device cannot beexceeded, the cost of material and labor for related support structurecan be reduced and a compact installation can be obtained.

It will occur to those skilled in the art, upon reading the foregoingdescription of the preferred embodiments of the invention, taken inconjunction with a study of the drawings, that certain modifications maybe made to the invention without departing from the intent or scope ofthe invention. It is intended, therefore, that the invention beconstrued and limited only by the appended claims.

We claim:
 1. In combination, a multi-storied structure having column andbeam frame members; earthquake-resisting braces secured to andreinforcing said frame members; non-variable, self-contained passivehydraulic damping devices secured between said frame members, or betweenone of said frame members and one of said earthquake-resisting braces,or between said earthquake-resisting braces on the individual stories ofsaid multi-storied structure, said passive hydraulic damping devicesbeing energized solely by seismic vibrations impacting on said framemembers to independently passively damp said seismic vibrations up to apredetermined level; and fail safe means to prevent vibration overloadon said non-variable, self-contained hydraulic damping devices, saidnon-variable, self-contained hydraulic damping devices havingpredetermined non-variable damping coefficients preselected and presetto provide damping factors within predetermined ranges.
 2. Thecombination of claim 1, wherein said steel frame members provide astructure having a stiffness factor within the range of thirty to onehundred percent.
 3. The combination of claim 1, wherein said dampingfactor is within the range of ten to forty percent.
 4. The combinationof claim 1, wherein one or more of said passive hydraulic dampingdevices are secured to one or more of said stories of said multi-storiedstructure and the said non-variable coefficients of damping of saidnon-variable passive hydraulic devices are selectively preset and fixedfor each of said stories to coordinate the overall damping effect ofsaid passive hydraulic damping devices on seismic vibrations.
 5. Thecombination of claim 4, wherein a plurality of said passive hydraulicdamping devices are secured to each of said stories of saidmulti-storied structure.
 6. The combination of claim 1, wherein saidmulti-storied structure has high, intermediate, and low modes of naturalvibration, and said non-variable coefficients of damping of saidnon-variable passive hydraulic damping devices are selectively presetand fixed to maximize damping of said intermediate mode of vibration. 7.In combination, a multi-storied structure having column-and-beam framemembers; earthquake-resisting braces secured to and reinforcing saidframe members; and passive hydraulic damping devices secured betweensaid frame members or between one of said frame members and one of saidearthquake-resisting braces or between said earthquake-resisting braces;said structure having natural modes of vibration V₁, V₂ and V₃ wherein adamping coefficient c₁ provides a maximum damping factor h₁, a dampingcoefficient c₂ provides a maximum damping factor h₂, and a dampingcoefficient c₃ provides a maximum damping factor h₃, in which therelationship of said coefficients is c₁ ≦c₂ ≦c₃ ; means to provide saidhydraulic damping devices with a fixed preset damping coefficient c₂ setat a predetermined level; means to preset said hydraulic damping devicesto prevent overloading by seismic vibrations exceeding a presetpredetermined level; and means to maintain said damping coefficient c₂at said fixed preset predetermined level.
 8. The combination of claim 7,wherein said column-and-beam type frame has a stiffness factor withinthe range of thirty to one hundred percent.
 9. The combination of claim7, wherein the said damping factor h₂ is within the range of ten toforty percent.
 10. The combination of claim 7, wherein said dampingdevice comprises a hydraulic cylinder having first and second pressureresponsive hydraulic chambers; means to permit hydraulic fluid to flowfrom said first hydraulic chamber to said second hydraulic chamberresponsive to a first level of seismic force; means to permit hydraulicfluid to flow from said second hydraulic chamber to said first hydraulicchamber responsive to a second level of seismic force; and said means toprevent said damping device from becoming overloaded by vibrationsincluding hydraulic relief valves between said first and secondhydraulic chambers.